DNA paternity testing of related potential fathers is not uncommon, although paternity in such cases is more difficult to prove beyond doubt. It is frequently possible if the relationship is known prior to testing, though in some such cases other evidence is generally required before paternity can be legally ascertained.
While positive DNA tests can be accepted as positive evidence of paternity in the vast majority of cases due the statistical improbability of potential fathers having the same DNA profiles, the situation is complicated if the possible fathers are related. It isn't uncommon for brothers to be involved in a parentage dispute, or even for the mother to have the DNA test carried out privately to determine which is the father of her child.
In normal paternity disputes, whether or not DNA profiling is acceptable as the sole evidence depends upon the circumstances of the case, and it is generally wise to be able to prove the likelihood, or even just the possibility, of the mother and the possible father coming into contact with one another. In Islamic law a paternity test is not sufficient, and other evidence, preferably that of witnesses, is necessary.
The Legal Aspects
Where DNA paternity testing is to be used in evidence, courts will generally insist on a legal paternity test where the collection of the sampling is carried out by an authorized professional and the identity of the subject legally determined. Home DNA testing, or DNA tests on samples presented by the subject, is not admissible as evidence. The probability of paternity accepted as definitive proof varies from state to state, although a probability of 99% would seem reasonable where there is no doubt as to the source of the sample, and where no complications such as rape are involved.
The fact that parentage has been discounted in 99 men in 100 is a more positive identification of the father where there are limited possibilities than were the father potentially any individual in the general population such as occurs with rape. In that case a probability of 99.9% or higher would be more definite, together with additional proof of the likelihood of the suspect being in the vicinity.
Such probability is determined largely by the accuracy of the test that depends upon both the way it is carried out and the number of DNA indicators used (a good average would be 16), and is frequently as high as 99.99% or more. It also depends on whether the sample of the Mother is included in the testing or not, as this will help to obtain a much higher level of probability.
Common DNA between Family Members
DNA tests on related potential fathers are no different from those carried out on any other subject: it is the results that could potentially create problems. Take the case of first cousins: they share 12.5% of their DNA and so it will be more difficult to prove parentage. This sharing of DNA increases to 25% for uncles, nephews and half-brothers, and to 50% for sons, fathers and brothers of the person being tested.
DNA paternity testing on different members of the same family is not an uncommon situation, and can occur for a number of reasons, including:
a) Parentage of pregnancies arising from consensual and non-consensual relationships, although the former is by far the more common of the two.
b) Two family members each claiming parentage, whether or not the mother is claiming one or neither to be the father.
c) A brother desiring confirmation that he is the parent.
d) The mother, or potential mother, having a private test to assure herself as to who is the father.
e) A grown child confirming parentage, particularly where an inheritance could be involved.
There are others, but these are common reasons for legal and home DNA tests being carried out where potential fathers could be related. Standard DNA paternity testing statistics are based upon unrelated members of the public, and unless the laboratory carrying out the DNA testing has been informed of the relationship, the test results could be misinterpreted.
The Case of Identical Twins
Nevertheless, it is possible to test additional DNA markers if any of the above relationships are known, but not in one specific case. This is the case of identical twins. Normal twins occur when two eggs are fertilized by two sperm, and are no more alike genetically than were they siblings born years apart. Identical twins, however, are formed when one egg is fertilized by one sperm and then splits into two halves within the first 14 days for no known reason.
These twins share an identical genetic make-up and so their DNA is identical and indistinguishable from one another. There is no test that can determine which is the biological father of a child. Unless one is excluded for some reason, either could be the father and that is all that DNA testing can prove. The court would have to disregard DNA results and use other means to determine paternity.
Old-fashioned policing, therefore, still has a part to play where the related potential fathers are identical twins. Apart from this specific case, DNA paternity tests are not entirely foolproof since they rely on probability, but are virtually conclusive where you simply want confirmation of paternity. In such a case, a likelihood of 1 in a 1000 is a virtual certainty and where confirmation is required between a limited number of candidates, a 99.9% DNA genetic testing result is legal confirmation.